In primary succession there is an event that will destroy everything on land like a volcano or a big fire. And after this event the land will be bare all you will see is racks. Then after many years plants will dart to go like weeds, then moss will grow then after many years bigger plants will start to go like shrubs and oak trees. Then the finale stage will be full-size trees will grow.
In secondary succession happen faster than primary succession. In secondary succession the soil is not disturb so the plants will grow much faster. So in this picture there is a fire and it will burn the trees. But the soil is fine so grass and moss will grow faster. After the moss there will be small shrubs, then there is big trees.